↑This shows an Alex Moulton small-wheeled bike fitted with an 11-speed Super Record rear derailleur,
shifted to the position closest to the condition where the center of the upper and lower pulleys passes through a vertical line from the ground,
in other words, where the lower pulley position is closest to the ground.
I made an effort to shoot it as much as possible from directly to the side
(though I suppose one could also shoot it more deliberately to make the rear derailleur appear closer to the ground).
The tires in the image show the marking "ETRTO 369mm," which refers to Moulton's proprietary 17-inch standard.

This, unlike in Japan with left-hand traffic, on right-hand traffic roads would be
likely to accidentally hit a curb while riding.

The pulleys on this rear derailleur are 11T, but whereas many manufacturers originally used 10T pulleys,
Shimano adopted 11T pulleys starting with the 7700 series Dura-Ace in road components.
After that, for more than 10 years Shimano used 11T pulleys,
Campagnolo used 10T pulleys, and with virtually no other manufacturers making road components besides these two,
third-party pulley makers like Tni and BBB labeled them as 11T for Shimano and 10T for Campagnolo.
Campagnolo also switched to 11T pulleys from the 11-speed rear derailleur onward.

The reason not to use 10T pulleys is that with an even number,
if the teeth engaging the outer link and inner link of the chain don't alternate,
the teeth on the inner link side wear more.
With 11T, being an odd number, this alternates with each pulley rotation.
I've seen the 10T pulleys on cheap commuter MTBs or cross bikes
that were used indefinitely on outer×top or middle×top
(↑the shifter serves no purpose)
worn down to the point where directly under the outer link, only about 5 teeth remained,
forming a five-pointed star shape, so odd-numbered pulleys certainly have the effect of preventing uneven wear.


↑This is one of the 9-speed 950 series XTUR rear derailleurs,
the RD-M953.


The sprocket tooth counts at that time were
12–34T, 11–34T, and 11–32T, three types,
about the same as current road tooth counts.
This rear derailleur's pulleys are 11T, but
with the maximum front chainring having 44T teeth and
current road (and MTB) rear derailleurs being single-tension,
the pulley spacing on the current Shimano 12-speed road rear derailleur
is longer than on this one.
And when mounted on the Moulton at the beginning,
under the worst conditions, the pulley cage comes
right to the ground (it scrapes when leaning in corners).

When the tooth count difference between sprockets and front chainrings increases,
meaning gear capacity increases, one method to address this is
to extend the distance between pulleys, but increasing the number of pulley teeth is also effective.
In fact, with the current XTUR... well, that's confusing since it just came out,
but both the current M9200 series and the M9100 series that was cutting-edge until recently
use 13T pulleys (also odd), so
the pulley spacing is actually shorter than the current road components,
which are also 12-speed, resulting in a counterintuitive outcome.
The M9200 series crankset is front single only,
and the M9100 series besides front single also has
a 38–28T front double, but with a tooth difference of 10T, smaller than road's maximum 16T,
which is also a factor in why the pulley spacing doesn't increase.


Incidentally, the point where this RD-M953
differs from the M950 by advancing the suffix model number
is that it's "low normal."
The spring in the parallelogram inside the rear derailleur
is at low gear when fully compressed,
opposite of most rear derailleurs in the world, and
when you pull the cable it shifts toward the top.
The advantage of this is that
shifting to a larger gear (upshifting) requires more force
than shifting to a smaller gear (downshifting),
but you can use the spring's compression force at that time, so
when downshifting to low gear on a climb,
it shifts snappily compared to top-normal rear derailleurs.
The disadvantage is that shifter operation
becomes reversed.

However, this is mechanical shifting and the shifter is shared, so
the operation is reversed; if it were electronic components,
couldn't you incorporate low-normal without making the rider aware of it
in the rear derailleur?
I thought, but with electronic components
the motor shifts forcefully, so
it doesn't matter.

Furthermore, I should mention that
there also exist front derailleurs with outer-normal
instead of inner-normal,
where pulling the cable shifts toward the inner side.
The RD-M953 also covers
considerably overlapping ground with this article
(→here), so please check that out.


↑This is the lower pulley of SRAM Rival 12-speed on my bike.
There's an engraving "U" on the upper pulley and "L" on the lower pulley
in one place, so I shot it there.
The pulley tooth count is 12T.
It's premature to think "even-numbered pulleys are stupid!"
because, remarkably, this pulley has
alternating tooth tip shapes for outer and inner link engagement,
the same tooth profile as so-called narrow-wide chainrings.


↑This is a rear derailleur from the 5500 series 105 of the 9-speed 7700 series Dura-Ace era.
The short cage model number ends in SS,
and the long cage for triple front is GS.
The only difference is the distance between pulleys;
both have 11T pulleys.

The 5501 designation indicates it's the successor to the 5500,
which had a bordered yellow 105 logo, and
it subsequently received a minor update to 5502.
With rear derailleurs, only the SS and GS suffixes differ between double and triple front versions, but
with cranksets the original 5500 series had FC-5500 for double and FC-5503 for triple,
which in the 5501 series became
FC-5501 and FC-5504, and in the subsequent 5502 series
FC-5502 and FC-5505, so
model numbers became sequential—I wondered
what they'd do for the next minor update,
but then the 10-speed 5600 series came out, so
the 5500 series didn't receive any further minor updates.
The 6500 series Altegra, the higher grade of this,
also maintained the 11T pulley approach on the triple-front rear derailleur
with a longer cage, but
at the even higher 7700 series front-triple rear derailleur
RD-7703....



As shown in the earlier link,
it uses upper and lower 13T pulley configuration,
accommodating front-triple capacity without excessively lengthening pulley spacing.
This is a product from more than 20 years ago, when so-called big pulley aftermarket parts were popular.
Shimano certainly understood the advantages of big pulleys,
and it's true that after considering both merits and demerits,
they haven't adopted them in current road components.

With current road components, the pulley cage comes
right to ground level as with the Moulton at the beginning, or even more so, so
I wish they'd make at least short-cage versions
that can mix and match with whatever the current multi-speed component is
...which is why I wrote this article.

Incidentally, Shimano does have a current-production rear derailleur
that claims to have shortened the pulley cage for small-wheeled bikes.
The CUES (Cues), available in 9, 10, and 11-speed versions,
comes in grades U8000 series, U6000 series, and U4000 series, and
as a variant of the lower-grade U4000 series,
the RD-U4010 rear derailleur
claims to be a short-cage model for 9 and 10-speed.
However, this isn't for the current top-tier 12-speed, and
it requires front gear as single (maximum 0T front gear tooth difference),
compatible only with 9-speed 11–36T and
10-speed 11–39T CUES, with total capacity only being
the larger tooth difference of 11–39T, which is 28T, and the pulleys are both 11T.
It's not like they made upper and lower 13T pulleys
to work with front doubles with a maximum tooth difference of 16T
in road components!

I'd like to see a model like "13T upper and lower pulley 12-speed Altegra-equivalent grade-outside-grade rear derailleur."

If the lateral movement of the rear derailleur per shifter operation
were identical on Shimano 12-speed road and MTB,
due to certain circumstances I can't write about here
the rear derailleurs from the old XTUR and XT that stubbornly refused to release—
released mechanical levers,
and 105, the only current 12-speed road component with mechanical levers, might be usable as a combination.

With electronic components, road uses wired,
MTB's new XTUR and others are wireless to avoid infringing on SRAM's e-Tap patent—
they couldn't release wireless until key patents expired,
so, for example, driving the XTUR rear derailleur with Dura-Ace levers
isn't possible.


Quoted from Shimano's official site explanation of the RD-U4010.
It claims the clearance between rear derailleur and ground increased 30%! but I wished they'd compared
at the positions where the upper and lower pulleys align vertically from the ground,
or closest to it.
That would be around the middle of the sprocket
(when shifting to low gear, the pulley cage lifts forward),
but in the image shown, the chain is on the top gear.
Also, in the image the rim standard looks unmistakably HE,
likely ETRTO 406 20-inch.
With ETRTO 305 16-inch HE rims
and shifting to where the pulley cage is closest,
the RD-U4010 also has considerably tight clearance.